A cataract refers to any opacity of the ocular crystalline lens. The normal crystalline lens is transparent, refractive, and provides adequate accommodation (shape change) to transmit and focus light on the retina at various distances. Accommodation in the human eye occurs through controlled changes in crystalline lens shape, thickness, and refractive surface placement relative to the cornea. With cataracts, the normally clear lens becomes cloudy, affecting the transmission of light through the lens and resulting in loss of visual acuity. Cataracts are most frequently associated with the normal aging process or pathology, but injury or mechanical violation of the ocular capsule surrounding the lens also causes cataract formation.
Because of the risk of inducing cataract formation, penetrating the lens capsule to introduce drugs or other agents into the lens has not been favorably considered.